We have modeled the photoinduced volume change in amorphous selenium. After photon absorption, we treated the excited electron and hole independently within the framework of the tight-binding formalism. We found covalent bond breaking in amorphous networks with photoinduced excited electrons, whereas excited holes contribute to the formation of interchain bonds. We also observed a correlated volume change of the amorphous samples. Our results provide a new and universal description, which can simultaneously explain the photoinduced volume expansion and shrinkage. This model is supported by very recent in situ surface height measurements for amorphous selenium.
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